Binary Output

A Binary Output models a physical two-state output, such as a relay that starts a fan or pump. A client writes ACTIVE or INACTIVE to Present_Value to switch the equipment. The object also records whether the hardware agrees with the command.

Typical uses

• Start and stop a supply fan
• Switch a pump relay on or off
• Open or close a motorised damper
• Energise a door strike
• Turn a heater stage on

Key properties

Property	What it holds
Present_Value	The commanded state, ACTIVE or INACTIVE. Clients write it at a priority from 1 to 16.
Priority_Array	Holds one command per priority. The highest-priority entry that is not NULL decides Present_Value.
Relinquish_Default	The state the output takes when every priority slot is empty.
Polarity	Says whether ACTIVE matches the energised or the de-energised hardware state.
Feedback_Value	The state the equipment reports back. A mismatch with Present_Value can raise an alarm.
Minimum_On_Time	How long the output must stay on before it may switch off again. Protects motors from short-cycling.

How it behaves

A Binary Output is always commandable. With intrinsic reporting, the standard pairs it with the COMMAND_FAILURE algorithm. The object goes OFFNORMAL when Feedback_Value disagrees with Present_Value for the time delay, and returns to normal when they agree again. Clients can subscribe to changes of Present_Value and Status_Flags.

With the CAS BACnet Stack

Enable it with the STACK_OPTION_OBJECT_TYPE_BINARY_OUTPUT build option and add each object with BACnetStack_AddObject. Present_Value is served through the callback you register with BACnetStack_RegisterCallbackGetPropertyEnumerated and the BACnetStack_RegisterCallbackSetPropertyEnumerated callback. Because the object is always commandable, the stack reads the priority array through the BACnetStack_RegisterCallbackGetPropertyBool callback (is the slot empty) and the BACnetStack_RegisterCallbackGetPropertyEnumerated callback (the value), with array indexes 1 to 16, and never calls the plain Present_Value read. Arm the alarm with BACnetStack_SetIntrinsicCommandFailureAlgorithm, after BACnetStack_SetAlarmsAndEventsForObjectEnabled.

Related object types

Binary Input, Binary Value, Binary Lighting Output, Multi-state Output

Properties

The properties ASHRAE 135-2024 clause 12.7 defines for this object type.

How to read the table:

• Conformance is what the standard asks of every object of this type: R is required, W is required and writable by a client, O is optional. A digit after the letter marks a condition in the standard that decides when the property is present or writable, such as support for intrinsic reporting.
• Present by default: yes means the stack presents the property as soon as you add the object; no means it is optional and you turn it on with BACnetStack_SetPropertyEnabled. This is the stack’s starting setup, so it can differ from the conformance code.
• Writable is whether a client can write the property by default. Change it with BACnetStack_SetPropertyWritable.
• not in profile means the stack does not offer this property for this object type.
• Value when your callback does not answer: “Held by the stack” means the stack stores and serves the value itself and does not ask your callbacks. Otherwise it is the value the stack answers when your callback declines or is not registered.

Property	Datatype	Conformance	Present by default	Writable	Value when your callback does not answer
Object_Identifier	BACnetObjectIdentifier	R	yes	no	Held by the stack.
Object_Name	CharacterString	R	yes	no	"Object <type>,<instance>", such as "Object 0,1" for Analog Input 1.
Object_Type	BACnetObjectType	R	yes	no	Held by the stack.
Present_Value	BACnetBinaryPV	W	yes	yes	Built by the stack from the priority array your callbacks serve, or Relinquish_Default when every slot is empty.
Description	CharacterString	O	no	no	""
Device_Type	CharacterString	O	no	no	""
Status_Flags	BACnetStatusFlags	R	yes	no	Built by the stack from your callbacks.
Event_State	BACnetEventState	R	yes	no	0
Reliability	BACnetReliability	O	no	no	0
Out_Of_Service	Boolean	R	yes	no	false
Polarity	BACnetPolarity	R	yes	no	0
Inactive_Text	CharacterString	O1	no	no	""
Active_Text	CharacterString	O1	no	no	""
Change_Of_State_Time	BACnetDateTime	O2	no	no	No default: the read fails.
Change_Of_State_Count	Unsigned	O2	no	no	0
Time_Of_State_Count_Reset	BACnetDateTime	O2	no	no	No default: the read fails.
Elapsed_Active_Time	Unsigned32	O3	no	no	0
Time_Of_Active_Time_Reset	BACnetDateTime	O3	no	no	No default: the read fails.
Minimum_Off_Time	Unsigned32	O	no	no	0
Minimum_On_Time	Unsigned32	O	no	no	0
Priority_Array	BACnetARRAY[16] of BACnetOptionalBinaryPV	R	yes	no	Built by the stack from your callbacks.
Relinquish_Default	BACnetBinaryPV	R	yes	no	No default: value-not-initialized until a callback or a client write supplies one.
Time_Delay	Unsigned	O4,6	no	no	0
Notification_Class	Unsigned	O4,6	no	no	0
Feedback_Value	BACnetBinaryPV	O4	no	no	0
Event_Enable	BACnetEventTransitionBits	O4,6	no	no	An empty bit string.
Acked_Transitions	BACnetEventTransitionBits	O4,6	no	no	An empty bit string.
Notify_Type	BACnetNotifyType	O4,6	no	no	0
Event_Time_Stamps	BACnetARRAY[3] of BACnetTimeStamp	O4,6	no	no	No default: the read fails.
Event_Message_Texts	BACnetARRAY[3] of CharacterString	O6	no	no	""
Event_Message_Texts_Config	BACnetARRAY[3] of CharacterString	O6	no	no	""
Event_Detection_Enable	Boolean	O4,6	no	no	false
Event_Algorithm_Inhibit_Ref	BACnetObjectPropertyReference	O6	no	no	No default: the read fails.
Event_Algorithm_Inhibit	Boolean	O6,7	no	no	false
Time_Delay_Normal	Unsigned	O6	no	no	0
Reliability_Evaluation_Inhibit	Boolean	O8	no	no	false
Property_List	BACnetARRAY[N] of BACnetPropertyIdentifier	R	yes	no	Held by the stack.
Interface_Value	BACnetOptionalBinaryPV	O	no	no	Null
Current_Command_Priority	BACnetOptionalUnsigned	R	yes	no	Built by the stack from your callbacks.
Value_Source	BACnetValueSource	O9,11,13	no	no	No default: the read fails.
Value_Source_Array	BACnetARRAY[16] of BACnetValueSource	O10,12	no	no	No default: the read fails.
Last_Command_Time	BACnetTimeStamp	O10,12	no	no	No default: the read fails.
Command_Time_Array	BACnetARRAY[16] of BACnetTimeStamp	O12	no	no	No default: the read fails.
Audit_Level	BACnetAuditLevel	O14	not in profile	not in profile	No default: the read fails.
Auditable_Operations	BACnetAuditOperationFlags	O14	not in profile	not in profile	No default: the read fails.
Audit_Priority_Filter	BACnetOptionalPriorityFilter	O14	not in profile	not in profile	No default: the read fails.
Tags	BACnetARRAY[N] of BACnetNameValue	O	no	no	No default: the read fails.
Profile_Location	CharacterString	O	no	no	""
Profile_Name	CharacterString	O	no	no	""